The man lied about his experience and forged a diploma from the officers' academy, according to the Dagens Nyheter newspaper.

He served in the Swedish Armed Forces for more than a decade and completed postings to high-security positions and international missions, including Afghanistan and last year to the UN peacekeeping force MINUSMA in Mali.

The Swedish Armed Forces conceded that the man had been "wrongfully employed" and confirmed in a statement he had served at NATO's Supreme Headquarters Allied Powers Europe (SHAPE) in Belgium.

"The person worked as a Swedish liaison officer and with an IT system used within the framework of those military operations where the Swedish Armed Forces has personnel," the statement said.

Following reports about the officer on Monday, the Swedish military said it had informed NATO and started an investigation.

Dagens Nyheter reported that he worked on the Afghanistan Mission Network -- a now-defunct system used by 48 nations for sharing intelligence, surveillance and reconnaissance information -- during 2012 and 2013 at NATO's HQ.

He rose to the rank of major despite not having completed mandatory training, and was even temporarily promoted to be able to serve at the military alliance headquarters, the newspaper said.

"These are serious allegations," NATO said in a statement to AFP on Wednesday.

"Staff on temporary NATO assignments are selected and sent by the home nation, which is also responsible for security clearances."

NATO added that it was in contact with Swedish authorities and it would be inappropriate to comment further on a case currently under investigation.

The Swedish military did not immediately respond to AFP's request for comment.

Sweden is not a member of NATO but has moved closer to the military alliance in recent years following Russia's annexation of Crimea.

In 2016, the government ratified an agreement making Sweden a so-called Host Nation of NATO, allowing the alliance to more easily operate on Swedish territory.